Mars: The watery shape of things to come

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  • Published: Feb 15, 2014
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Mars: The watery shape of things to come

No ocean in motion

Dark flow like features called Recurring Slope Lineae emanating from bedrock exposures at Palikir crater on Mars during southern summer. These flows are observed to form and grow during warm seasons when surface temperature is hot enough for salty ice to melt, and fade or completely disappear in cold season. Arrows point to bright, smooth fans left behind by flows.

Mars watchers have known since around 2011 that there exist features on the Red Planet's surface that may well be the result of the movement of water. The streaks appear and disappear periodically and are referred to as recurring slope lineae (RSL). A new study has examined 13 confirmed RSL sites on the Martian surface using images from the compact reconnaissance imaging spectrometer for Mars (CRISM). Unfortunately, the spectral lines did not reveal the definitive signature of water nor of salts that might be dissolved in the medium. However, they do hint at distinct and consistent spectral signatures of iron(III) - ferric - and iron(II) - ferrous - minerals at most of the RSL sites, a tantalizing paddle towards aqueous evidence.

Lujendra Ojha (now at Georgia Institute of Technology) discovered RSLs while an undergraduate at the University of Arizona, these features appear annually and occur on the steep slopes of crater walls. Identifying the minerals they might leave in their wake could be key to understanding what they are and whether or not they are water-based.

Martian antifreeze

Now, Ojha working with James Wray suggest that the presence of ferric and ferrous mineral grains offers a clue regarding the nature of RSL-related materials as opposed to non-RSL slopes. Ohja suggests they do not yet have a "smoking gun" for the existence of water in RSLs, nor even a steaming kettle, although explaining the phenomenon without invoking water as the source would be difficult." Just like the RSL themselves, the strength of the spectral signatures varies according to the seasons," explains Ojha." The signatures are stronger when it’s warmer and less significant when it's colder."

The research team also notes that the lack of water-related absorptions rules out hydrated salts as a spectrally dominant phase on RSL slopes. For example, ferric sulfates have been found elsewhere on Mars and are known to behave like a strong antifreeze. If such salts are present in RSLs, then they must be dehydrated considerably under exposure to the planet’s conditions by the time CRISM observes them in the mid-afternoon. The team has published details in the journals Geophysical Research Letters and Icarus. Their conclusion in essence suggests that predicting where RSL will appear is, at best, a guessing game.

Following the water

The team, working with colleagues in Arizona looked at all the images gathered by the High Resolution Imaging Science Experiment (HiRISE) between March and October of 2011 hoping to find the ideal hunting grounds for RSLs, areas near the southern mid-latitudes on rocky cliffs. They identified 200 few of these had RSLs. “Only 13 of the 200 locations had confirmed RSL,” Ojha explains. "There were significant differences in abundance and size between sites, indicating that additional unknown factors such as availability of water or salts may play a crucial role in RSL formation." They have also now compared the latest observations with earlier images, the team also found that RSL are much more abundant some years than others. If RSLs contain water, then water on Mars today is much more elusive than it has been in the past.

NASA likes to "follow the water" in exploring the Red Planet, for obvious reasons. Water is crucial to life as we know and so if there is a chance that life existed on Mars then it could only do so if there were water there. Additionally, if human are to make Mars a second home, it would be useful to know that there is a ready water supply. "We'd like to know in advance when and where it will appear," Wray explains. "RSLs have rekindled our hope of accessing modern water, but forecasting wet conditions remains a challenge."

"What we'd ultimately like to do is first prove, or disprove, that these features are formed by liquid water!" Wray told SpectroscopyNOW. Secondly, if they are water features, he adds that they would like to , "figure out where the water comes from (subsurface ice or atmospheric water vapour and understand the liquid water's chemistry to assess whether it is habitable." If it is then he hopes that it might be possible to go there and find out if Martian microbes have made a home there. "While landing on RSL would allow us to answer all of these questions, a simpler next step would be to observe them at a different time of day," he explains. "All our CRISM and HiRISE images to date have been taken at ~3pm, but liquid is most stable in the early morning so by mid-afternoon the RSL have likely dried out considerably.  A new orbiter that could make spectral observations at the time of peak water stability might allow us to at least answer our first question." 

Related Links

Geophys Lett, 2014, 40, 5621-5626: "Spectral constraints on the formation mechanism of recurring slope lineae" 

Icarus, 2014, 231, 365-376: "HiRISE observations of Recurring Slope Lineae (RSL) during southern summer on Mars" 

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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